The shadow mask concept currently used in color CRTs dates back to 1949. While the mask pattern has taken on various forms over the years such as dots, strips and slots, the basic theory of operation remains unchanged: three separately modulated electron beams converged and scanned both horizontally and vertically across a cathodoluminescent screen by means of a deflection yoke mounted on the CRT neck are used. Display panel screening is made photolithographically using a mask as the stencil. Shadow mask color CRTs have dominated the consumer market for more than four decades because of their far superior brightness, contrast and mature technologies.
The shadow mask is used in combination with a target or screen consisting of a regular pattern of photo-deposited triads of red, blue and green light-emitting phosphors on the CRT's faceplate. The shadow mask is foraminous and is disposed a predetermined distance from the target, and by virtue of its pattern of beam passing apertures, effectively shadows all but selected ones of the individual light-emitting phosphors from its corresponding electron beam-emitting source located in the neck of the CRT. Precise registration between the mask's beam passing apertures and the faceplate's light-emitting phosphor deposits is essential for a high degree of video image color purity. Misregistration of the mask apertures with the phosphor deposits is sometimes caused by mask "doming" caused by nonuniform electron beam heating and expansion of the mask. The prior art includes various proposed approaches for correcting for shadow mask doming as disclosed, for example, in U.S. Pat. Nos. 4,629,932; 4,656,388; 4,665,338; 4,716,333; 4,734,615 and 5,028,836. Maintaining precise registration between the shadow mask apertures and the faceplate phosphor deposits is even more critical, and more difficult to maintain, in high definition television (HDTV) receivers which incorporate a flat shadow mask maintained in a stretched condition under high tension.
Misregistration of the shadow mask apertures with the faceplate phosphor deposits may also arise from vibration of the shadow mask. Shadow mask vibration is typically caused by extraneous factors such as by impact with the faceplate or high intensity sound waves as produced by high quality audio signals in television receivers equipped with a stereo receiving capability. Shadow mask vibration becomes increasingly severe for shadow masks having reduced curvature and finer pitch (increased number of beam passing apertures per unit area) such as employed in high performance color monitors and high end, large display television receivers. Shadow mask damping is also critical because of the increasing use of materials having a low modulus of elasticity and high yield strength which are more subject to vibration, particularly at low frequencies, and particularly in the case of masks comprised of Invar. Even masks having a higher modulus of elasticity such as those comprised of aluminum killed (AK) steel exhibit vibration. Invar is comprised of an iron-nickel alloy having a small coefficient of thermo-expansion, while AK steel is steel to which a strong de-oxidizing agent (such as aluminum) has been added while in the molten state to minimize the reaction between oxygen and carbon during solidification. In addition, vibration of the shadow mask may cause a coating such as of graphite on the mask to separate and minute flakes to fall off. Flakes adhering to the shadow mask may cause blockage of the electron apertures, adversely affecting the characteristics of the video image on the phosphor screen. Loosened flakes adhering to the electron gun may cause sparks between the electrodes, limiting the capacity to withstand high voltages and also contributing to a reduction in video image quality.
The present invention addresses the aforementioned limitations of the prior art by providing shadow mask damping for a color CRT which restricts mask vibration for improved video image color purity.